December 2012

Is the tobacco plant Nicotiana alata the key to saving crops from fungus?

NaD1 ribbon structure
NaD1 ribbon structure. Image courtesy of Mark Hulett.

When you think of tobacco, you might be reminded of its negative health effects. While this association holds true for cigarette tobacco, ornamental tobacco is grown for decorative purposes and often recognized for its striking flowers and fragrance. Recently, the high-resolution crystal structure of the plant defensin NaD1, derived from the flowers of the ornamental tobacco plant Nicotiana alata, was solved and published in The Journal of Biological Chemistry.

“They’re unique structures,” says Mark Hulett, research director at La Trobe Institute for Molecular Science and the lead researcher on the study. “NaD1 is a small peptide that is highly disulfide-bonded … and that makes it incredibly stable.” This stability may partially explain why defensins are able to permeabilize fungal membranes, a step required for fungal cell death.

Using cross-linking experiments, small-angle X-ray scattering analysis and analytical centrifugation, the group demonstrated that NaD1 forms dimers in solution under physiological conditions. It has been proposed that the formation of dimers or higher-order oligomers in antimicrobial peptides contributes to their ability to disrupt biological membranes, which appears to be critical for killing fungus. The JBC paper is the first report of plant defensins dimerizing.

Nicotiana alata flower
Nicotiana alata flower. Image courtesy of WikiCommons.

Furthermore, site-directed mutagenesis of the Lys4 amino acid residue revealed an important functional link between NaD1 dimerization and antifungal activity. This NaD1 mutant was unable to form dimers and displayed a greatly reduced capacity to kill the filamentous fungi F. oxysporum, presumably due to its inability to permeabilize the fungal membrane. According to the paper, the dimerization of NaD1 results in the formation of a large positively charged surface that may be important for mediating interactions with the negatively charged surfaces of the fungal cell wall and plasma membrane. It is believed that this interaction facilitates cell permeabilization of the fungal pathogen.

While the results of this study shed light on the molecular mechanism behind how defensins may recognize and penetrate the cell membranes of fungi, the intracellular targets of the molecule are unclear. “We know that defensin permeabilizes the plasma membrane and gets into the cytoplasm of the fungus,” says Hulett. “We propose that it’s hitting intracellular targets. Those targets we’re not sure of at this stage. But we do think that those interactions are probably disrupting various signaling pathways and important processes of the fungus.”

Fungal pathogens cause devastating crop losses worldwide, particularly of corn and wheat. Unfortunately, fungicides have been only partially successful in controlling these losses. The naturally derived NaD1 has been shown to play a key role in defending against fungal pathogens. The tobacco plant Nicotiana alata makes high levels of this molecule, and this study reveals that the ornamental plant actually may have major implications in agrobiotechnology.

Shannadora HollisShannadora Hollis ( received her B.S. in chemical engineering from North Carolina State University and is a Ph.D. student in the molecular medicine program at the University of Maryland, Baltimore. Her research focuses on the molecular mechanisms that control salt balance and blood pressure in health and disease. She is a native of Washington, D.C., and in her spare time enjoys cooking, thrift-store shopping and painting.

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